Method of forming bimetallic transition joints

ABSTRACT

Transition joints formed of dissimilar metals, such as stainless steel and aluminum, are produced by assembling an aluminum rod and two stainless steel plugs coaxially within a stainless cylinder with the plugs positioned at each end of the rod. Both plugs are welded to the cylinder and the assembly is then heated and subjected to reduction by rotary swagging thus forming a metallurgical bond of the two metals. The assembly may be machined into tubular joints by standard techniques.

United States Patent Inventor Appl. No. Filed Patented Assignec METHODOF FORMING BIMETALLIC 2,820,751 l/1958 Saller 29/471.3 3,100,742 8/1963McGeary et al. 29/474.3 X 3,160,951 12/1964 Markert et a1 29/474.33,245,140 4/1966 Markert, Jr. et a1. 29/473.3X 3,429,025 2/1969 Baily eta1 29/474.3 X

Primary Examiner-John F. Campbell Assistant Examiner-Richard BernardLazarus Attorneys-R. S. Sciascia and A. L. Branning swagging thusforming a metallurgical bond of the two metals. The assembly may bemachined into tubular joints by standard techniques.

TRANSITION JOINTS 7 Claims, 4 Drawing Figs.

U.S. Cl 29/471.1, 29/474.3, 29/480, 29/481 Int. Cl 823k 31/02 FieldofSearch 29/474.3,

References Cited UNITED STATES PATENTS 1,753,638 4/1930 Axtel] 29/481PMENTED SEP 7 B?! Sn QFx hump-1! INVENT OR WILL/A D. OAKS ATTORNEYSMETHOD OF FORMING BIMETALLIC TRANSITION JOINTS STATEMENT OF GOVERNMENTINTEREST The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates generally to metalworking and more particularly to a method of fabricating a gasimpervious tubular connection between two dissimilar metals which do notreadily lend themselves to coupling by conventional techniques such aswelding.

Certain metals do not lend themselves to being welded together byconventional techniques and therefore, it has been the practice in thepast to join such dissimilar metals by means of mechanical devices suchas flarings, farrels, flanges, and O- ring-type fittings. Couplingsfabricated by these techniques do not perform satisfactorily under alloperating conditions, particularly in those situations where thecoupling is subjected to cryogenic temperatures. When exposed tocryogenic temperatures, the differential expansion of the dissimilarmetals will often cause metal-to-metal seals to fracture and leak, therings become glass hard and thereby lose the effectiveness of the seal,and organic sealants such epoxies will become hard and fracture easilywith slight vibrations. Since conventional mechanical couplingsconsistently fail when subjected to cryogenic temperatures, the methodof this invention was conceived for joining together dissimilar metalsin a manner to provide a gas pervious tubular coupling suitable foroperation over wide temperature ranges including cryogenic temperatureswhile maintaining the gastight integrity of the system.

SUMMARY OF THE INVENTION The general purpose of the present invention isto provide a method for forming bimetallic transition joints whichembraces all of the advantages of previous methods and yet possessesnone of the aforedescribed advantages. To attain this, the presentinvention contemplates the assembly of an aluminum rod and two stainlesssteel plugs within a close fitting stainless steel cylinder, the plugsbeing positioned at each end of the aluminum rod and in coaxialalignment therewith. Each stainless steel plug is then welded to thestainless steel cylinder to thereby enclose the aluminum rod in agastight envelope of stainless steel. The assembly is then heated andsubjected to a reduction by rotary swagging which causes the aluminumand stainless steel to grow together at their interface and form a soundmetallurgical bond therebetween. Upon cooling, the assembly may bemachined by conventional techniques to produce tubular transition jointshaving a sound metallurgical bond between the two metals forming thejoint. Transition joints formed in accordance with the method of thisinvention have been found to remain impervious to gas and thus preventleaks even when operated over wide temperature differentials includingcryogenic temperatures.

OBJECTS OF THE INVENTION It is therefore an object of the presentinvention to provide a method of forming improved transition joints foruse in the coupling of dissimilar metals.

Another object of the present invention is to provide an improvedmetallurgical bond between dissimilar metals having substantiallydifferent coefficients of thermal expansion.

A further object of the present invention is to provide a method offabricating an improved tubular transition joint having a soundmetallurgical bond between the two metals forming the joint and beingfree of gas leakage over wide ranges of temperature operation.

BRIEF DESCRIPTION OF THE DRAWINGS joints which may be machined from theassembly shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Fig. l, thestarting materials necessary for carrying out the method of thisinvention are shown in their assembled form. These starting materialsinclude a stainless cylinder 11, and aluminum rod 12, and two stainlesssteel plugs 13 and 14. As shown, the aluminum rod is inserted within thestainless cylinder and a stainless plug is partially received withineach end of the cylinder at opposed ends of the aluminum rod. Prior toassembly, these four parts are carefully machined for an accurate fit;the stainless steel cylinder being prepared by sizing its inner diameterto a smooth round bore by means of honing while the aluminum plug andthe stainless steel plugs are prepared by turning their outer diametersto a smooth round surface which may be for example 0.001 inch smallerthan the inner diameter of the cylinder. Such machining eliminatessurface irregularities on the parts and assures a close fit of all partsin the assembly.

For optimum results, it is also desirable to thoroughly clean the fourparts prior to assembly. The cleaning of the stainless steel parts maybe accomplished by dipping these parts in trichloroethylene to achieve avapor degrease, drying, dipping these parts in an alkaline solution,drying, dipping these parts in an acid solution such as a mixture ofnitric acid and hydrofluoric acid, drying, dipping in cold water rinseand drying by blowing air on the parts. Cleaning of the aluminum partsmay be accomplished by dipping the aluminum rod in trichloroethylene forvapor degrease, drying, dipping in alkali solution and subjecting to acold water rinse, dipping the rod into an acid solution such as dilutenitric acid and subjecting to a cold water rinse, dipping the rod into azincate solution 7 and subjecting to a cold water rinse, repeating theacid dip and cold water rinse and then repeating the zincate dip andcold water rinse and finally drying the parts by blowing air on the rod.Cleaning the parts in this manner will remove surface contaminations,will eliminate the absorbed and adsorbed gases on the surfaces of theparts and will remove the oxide coating which is otherwise present onthese parts. The cleaning steps eliminate the contaminating coatingswhich might otherwise inhibit the formation of the metallurgical bondbetween the metals which is desired to be achieved.

After the parts have been cleaned and assembled in the manner shown inFig. 1, the stainless steel rod 13 is welded to the stainless steelcylinder 11 in a manner to produce a continuous annular weld joint 15 atthe juncture of the end of the stainless steel rod with the plug.Similarly, a second annular weld joint 16 is produced at the juncture ofplug 14 with the other end of the cylinder 11. Welding stainless steelparts in the manner described serves to completely enclose the aluminumrod 12 within a continuous stainless steel jacket or envelope.

The welded assembly is then heated to a point above the recrystalizationtemperature, but below the lowest melting point of any metal in theassembly and subjected to a reduction in cross-sectional area. Thisreduction is preferably accomplished by means of rotary swagging whichproduces very high internal pressures in the assembly. These highinternal pressures in the assembly. These high internal pressures, ac-

companied by flexing of the tube walls during the rotary swaggingoperation, produces a very good metallurgical bond throughout the areasof contact of the aluminum with the stainless steel. It has been foundthat excellent metallurgical bonds of the dissimilar metals are producedby a 50 percent reduction of cross-sectional area, although lesseramounts of reduction of crosswill produce satisfactory results. Althoughiron, which is a constituent of stainless steel, and aluminum are twometals which chemically react to produce an intermetallic compound whichhas brittle characteristics, such a compound has not been detected inthe bonds produces in accordance with this method even thoughtheoretically such a layer should exist.

The quality of the metallurgical bond between the dissimilar metalsproduced by the method of this invention is enhanced by the particulararrangement of parts and assembly of parts which enables all entrappedair which might be present between the outer diameter of the aluminumplug and the inner diameter of the stainless steel cylinder to bedisplaced from this area. Any air present at the interface of thealuminum and stainless steel parts is permitted to be displaced from theinterfacial area toward the welded areas 15 and 16 by the pressuresexerted thereon during the swagging operation. The temperature andpressure conditions during the swagging operation are not sufficient toproduce a bond between the stainless steel plugs and the stainless steelcylinder, which would require much higher temperature and pressure. ThusWHATEVER AIR IS PRESENT IN THE ASSEMBLY IS PERMITTED TO COLLECT IN THESTAINLESS STEEL TO STAINLESS STEEL INTERFACE AREAS -& AND WHILE THEMETALLURGICAL BOND OF THE ALU- MINUM TO STAINLESS STEEL IS BEINGCREATED. The chromium oxide layer on the stainless steel cylinder isbelieved to give up its oxygen by denying it oxygen, i.e., removing theair at the aluminum and stainless steel interface, thus preventing anoxide layer from inhibiting the metallurgical bond which is beingproduced. Since the temperatures employed in the method of thisinvention are well below the melting point of any of the metals beingworked, the intermetallic layer is kept very thin.

Referring now to FIG. 3A, there is shown a bimetallic transitionmanufactured from the assembly shown in FIG. 2. This transition joint isformed by cutting away the stainless steel plugs 13 and 14 from theswaged assembly shown in FIG. 2, making a transverse cut through theassembly at a point midway between the ends of the aluminum rod,drilling an axial bore throughout the length of the remaining workpiece,milling off the stainless steel layer from the left end of the workpiecealong a portion which is less than one-half the length of the workpiece,and drilling out the aluminum from the right end of the workpiece to adepth less than one-half the length of the workpiece, to thus produce atransition joint having a tubular aluminum section 22 joined to atubular stainless steel section 21 by means of a lap joint 23 in whicharea the stainless steel and aluminum overlap and are metallurgicallybonded together.

Referring now to FIG. 3B, there is shown a transition joint formed by analuminum section 32 and a stainless steel section 31 joined together bya butt joint 33. This transition joint is formed by making a transversecut through the assembly at a point along the length of the aluminum rod12. The remaining workpiece is then milled down to the final diameter ofthe aluminum rod 12 thus removing the stainless steel layer includingthe welds l and 16. Then by forming an axial bore throughout the lengthof the workpiece, the transition joint shown in FIG. 3B is produced withthe aluminum and stainless steel tubular sections metallurgically bondedtogether by a butt joint.

From the foregoing, it will be apparent that applicant has provided anew and improved method of producing improved tubular transition jointsformed of dissimilar metals which cannot be welded together byconventional techniques. By practicing the method hereinabove described,dissimilar metals may be joined together by a metallurgical bond to formtubular transition joints which will remain free from gas leakage whenoperated over extreme ranges of temperature variation, includingcryogenic temperatures. Moreover, the method of this invention may bepracticed with relatively inexpensive equipment commonly found in mostmachine shops.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that the invention may be practiced otherwise than asspecifically described. For example, transition joints may bemanufactured in large quantities in accordance with the method of thisinvention by merely utilizing a longer stainless steel cylinder 11within which are alternately positioned a plurality of aluminum andstainless steel rods in the area between the stainless steel plugs 13and 14 which are welded to the cylinder.

What is claimed and desired to be secured by letters patent of theUnited States is:

1. A method of joining two metals having different coefficients ofthermal expansion comprising:

positioning a rod formed of a first metal within a cylinder which isformed of a second metal and is longer than said rod, positioning firstand second plugs formed of said second metal at least partially withinopposed ends of said cylinder and in abutting contact with said rod,welding each plug to said cylinder, and forming a metallur gical bondbetween said two metals by heating the assembled rod, plugs andcylinder, and reducing the cross-sectional area of said assembly by 10to 50 percent, severing said first and second plugs and a portion ofsaid rod and said cylinder to'form a workpiece comprising the remainingportion of said rod and said cylinder. 2. The method of claim 1 wherein:the reducing step is performed by rotary swagging until the desiredamount of reduction is achieved. 3. The method of claim 2 wherein: therods and plugs are machined to smooth close fitting surfaces prior toassembly. 4. The method of claim 2 wherein the welding step includes:making a continuous annular weld around each plug to join the plugs tothe cylinder. 5. The method of claim 3 wherein the welding stepincludes: making a continuous annular weld around each plug at aposition thereon which is spaced from the rod. 6. A method of joiningtwo metals having different coefficients of thermal expansioncomprising:

positioning a rod formed of a first metal within a cylinder which isformed of a second metal and is longer than said rod, positioning firstand second plugs formed of said second metal at least partially withinopposed ends of said cylinder and in abutting contact with said rod,welding each plug to said cylinder, and forming a metallurgical bondbetween said two metals by heating the assembled rod, plugs andcylinder, and reducing the cross-sectional area of said assembly by 10to 50 percent, cooling the assembly subsequent to the rotary swaggingcutting away the original cylinder and the outer portion of the plugs sothat the final diameter of the assembly is reduced to the final diameterof the rod, drilling an axial bore throughout the length of theassembly,

and transversely cutting the drilled assembly at a position between theends of the rod portion to thereby form two tubular transition jointseach having tubular sections of dissimilar metals joined together by ametallurgical bond at a butt joint. 7. A method of joining two metalshaving different coefficients of thermal expansion comprising:

positioning a rod formed of a first metal within a cylinder which isformed of a second metal and is longer than said rod,

positioning first and second plugs formed of said second metal at leastpartially within opposed ends of said 5 cylinder and in abutting contactwith said rod,

welding each plug to said cylinder, and

forming a metallurgical bond between said two metals by heating theassembled rod, plugs and cylinder, and

reducing the cross-sectional area of said assembly by to 10 50 percent,

cooling the assembly subsequent to rotary swagging transversely cuttingthe assembly near the midpoint of the rod to make two sections,

drilling an axial bore in one end of each section to a depth anddiameter so as to remove all of the rod along a portion thereof lessthan one-half the length of the section,

cutting away the second metal from the outer diameter of the other endof each section along a portion thereof which is less than one-half thelength of the section and,

drilling an axial bore through the sections to form two transitionjoints each having two tubular sections of dissimilar metals joinedtogether by a lapjoint at which is formed a metallurgical bond.

2. The method of claim 1 wherein: the reducing step is performed byrotary swagging until the desired amount of reduction is achieved. 3.The method of claim 2 wherein: the rods and plugs are machined to smoothclose fitting surfaces prior to assembly.
 4. The method of claim 2wherein the welding step includes: making a continuous annular weldaround each plug to join the plugs to the cylinder.
 5. The method ofclaim 3 wherein the welding step includes: making a continuous annularweld around each plug at a position thereon which is spaced from therod.
 6. A method of joining two metals having different coefficients ofthermal expansion comprising: positioning a rod formed of a first metalwithin a cylinder which is formed of a second metal and is longer thansaid rod, positioning first and second plugs formed of said second metalat least partially within opposed ends of said cylinder and in abuttingcontact with said rod, welding each plug to said cylinder, and forming ametallurgical bond between said two metals by heating the assembled rod,plugs and cylinDer, and reducing the cross-sectional area of saidassembly by 10 to 50 percent, cooling the assembly subsequent to therotary swagging cutting away the original cylinder and the outer portionof the plugs so that the final diameter of the assembly is reduced tothe final diameter of the rod, drilling an axial bore throughout thelength of the assembly, and transversely cutting the drilled assembly ata position between the ends of the rod portion to thereby form twotubular transition joints each having tubular sections of dissimilarmetals joined together by a metallurgical bond at a butt joint.
 7. Amethod of joining two metals having different coefficients of thermalexpansion comprising: positioning a rod formed of a first metal within acylinder which is formed of a second metal and is longer than said rod,positioning first and second plugs formed of said second metal at leastpartially within opposed ends of said cylinder and in abutting contactwith said rod, welding each plug to said cylinder, and forming ametallurgical bond between said two metals by heating the assembled rod,plugs and cylinder, and reducing the cross-sectional area of saidassembly by 10 to 50 percent, cooling the assembly subsequent to rotaryswagging transversely cutting the assembly near the midpoint of the rodto make two sections, drilling an axial bore in one end of each sectionto a depth and diameter so as to remove all of the rod along a portionthereof less than one-half the length of the section, cutting away thesecond metal from the outer diameter of the other end of each sectionalong a portion thereof which is less than one-half the length of thesection and, drilling an axial bore through the sections to form twotransition joints each having two tubular sections of dissimilar metalsjoined together by a lapjoint at which is formed a metallurgical bond.